docs: def format(format_string, cast=lambda x: x):
      """
      A pre-called helper to supply a modern string format (the kind with {} instead of %s), so that
      it can apply to each value in the column as it is rendered.  This can be useful for string
      padding like leading zeroes, or rounding floating point numbers to a certain number of decimal
      places, etc.
  
      If given, the ``cast`` argument should be a mapping function that coerces the input to whatever
      type is required for the string formatting to work.  Trying to push string data into a float
      format will raise an exception, for example, so the ``float`` type itself could be given as
      the ``cast`` function.
  
      Examples::
      
          # Perform some 0 padding
          item_number = columns.FloatColumn("Item No.", sources=['number'],
                                            processor=format("{:03d}))
  
          # Force a string column value to coerce to float and round to 2 decimal places
          rating = columns.TextColumn("Rating", sources=['avg_rating'],
                                      processor=format("{:.2f}", cast=float))
  
      """
  
      def helper(instance, *args, **kwargs):
          value = kwargs.get('default_value')
          if value is None:
              value = instance
          value = cast(value)
          return format_string.format(value, obj=instance)
      return helper